lighting ratio

Definition: Comparison of the brightness of main lumination on a subject with the secondary, less bright, lumination.
The stuff of portrait school
Lighting ratio, is strictly speaking, not a comparison of the brightness of the light-sources themselves, but their effect on the subject. That is, lighting ratio is not luminance ratio (ratio of luminance brightest bit to luminance of darkest bit) -- but in practice, there's not too much difference. An incident light meter reading gives a good enough guide to lighting ratio.

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